HooksMaster: Make RunPhase return the rpc output

Right now the hooks output is propagated from the nodes all the way up to
HooksMaster.RunPhase, which uses it for debugging PRE hooks, but then silently
discards them. We'll now propagate it up to the Processor.ExecOpCode function,
where they can be handled for other purposes (or discarded again, of course).
This patch also improves a bit the HooksMaster.RunPhase docstring.

Reviewed-by: iustinp

lib/mcpu.py

@@ -264,6 +264,12 @@ class HooksMaster(object):
 
     This is the main function of the HookMaster.
 
+    Args:
+      phase: the hooks phase to run
+
+    Returns:
+      the result of the hooks multi-node rpc call
+
     """
     if not self.node_list[phase]:
       # empty node list, we should not attempt to run this as either
@@ -287,6 +293,7 @@ class HooksMaster(object):
             errs.append((node_name, script, output))
       if errs:
         raise errors.HooksAbort(errs)
+    return results
 
   def RunConfigUpdate(self):
     """Run the special configuration update hook